An analog-digital converter of the kind specified is known from P. T. Callahan, M. L. Dennis, T. R. Clark: Photonic analogue to digital conversion, John Hopkins APL Technical Digest, vol. 30, no. 4 (2012) 280. This analog-digital converter essentially carries out two functions. On the one hand, the amplitude of the analog input signal is sampled in discrete steps. On the other hand, the time profile is quantized at a predefinable sampling rate. According to the Nyquist Theorem, the sampling rate of the analog signal must be at least twice the maximum frequency of the analog input signal in order to allow for an unambiguous reconstruction of the analog input signal from the digital data. The amplitude is quantized in a predefinable number of steps at the times defined by the sampling rate. For example, an analog-digital converter having a resolution of eight bits is able to distinguish 256 steps of the amplitude. Errors during the analog/digital conversion occur in particular due to a resolution of the amplitude which is too low or due to errors in the quantization of the time axis. The higher the bandwidth of the input signal is, the greater the accuracy with which the quantization of the time axis must be carried out. Thus, conventional analog-digital converters operating purely electronically reach their limits, in particular in the case of high frequencies of the analog signal and/or high accuracy requirements.
The aforementioned related art therefore provides for modulating the electrical analog input signal onto an optical carrier, converting this modulated signal once again into an electrical signal by means of a photodiode, and supplying it to a known electronic analog-digital converter. A short-pulse laser is used as an optical carrier signal, which generates pulsewidths of less than one picosecond. These laser pulses having a constant amplitude are supplied to an electro-optical modulator which is controlled by the analog input signal. A pulse train is available at the output of the electro-optical modulator, in which the time interval of the individual pulses represents the quantization of the time axis, and the amplitude of the input signal is encoded onto the optical carrier signal as an amplitude variation of the individual pulses. Due to the high time constancy of the laser, the quantization of the time axis is able to be carried out at a high frequency. The optical signal is distributed to multiple parallel analog-digital converters using an optical switch having a high switching frequency, so that a multiple of the bandwidth is achieved with respect to a purely electrical analog/digital conversion.
However, this known analog-digital converter has the disadvantage that the resolution of the downstream electronic analog-digital converter limits the available resolution.